Gate-All-Around Nanowire MOSFET and Method of Formation

ABSTRACT

A method for fabricating a semiconductor device comprises forming a nanowire on an insulator layer at a surface of a substrate; forming a dummy gate over a portion of the nanowire and a portion of the insulator layer; forming recesses in the insulator layer on opposing sides of the dummy gate; forming spacers on opposing sides of the dummy gate; forming source regions and drain regions in the recesses in the insulator layer on opposing sides of the dummy gate; depositing an interlayer dielectric on the source regions and the drain regions; removing the dummy gate to form a trench; removing the insulator layer under the nanowire such that a width of the trench underneath the nanowire is equal to or less than a distance between the spacers; and forming a replacement gate in the trench.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/532,122, filed on Nov. 4, 2014, which is a continuation of U.S.patent application Ser. No. 14/046,069, filed on Oct. 4, 2013, which isa continuation of U.S. patent application Ser. No. 14/035,060, filed onSep. 24, 2013, the contents of all of the foregoing applications beingincorporated herein by reference in their entireties.

BACKGROUND

The exemplary embodiments of this invention relate generally tosemiconductor devices and, more particularly, to a complementary metaloxide semiconductor device having a gate all-around nanowire structure.

A complementary metal oxide semiconductor device (CMOS) usessymmetrically-oriented pairs of p-type and n-type metal oxidesemiconductor field effect transistors (MOSFETs) arranged on silicon orsilicon-on-insulator (SOI) substrates. Source and drain regionsassociated with each MOSFET are connected by a channel. A gate disposedadjacent the channel controls the flow of current between the source anddrain regions. The channel may be defined by a thin “fin” or otherstructure that provides surface(s) through which the gate controls theflow of current.

In a device such as a gate-all-around (GAA) nanowire MOSFET, the channelbetween the source and drain regions is a nanowire, and the gatesurrounds the nanowire. Formation of such a gate so as to wrap aroundthe nanowire is a challenge. For example, it is often difficult (if notimpossible) to pattern the portion of the gate that is locatedunderneath the nanowire. Hence, the bottom portion of the gate maybecome longer than the top portion and may overlap the source and drainregions of the device.

In fabricating a GAA nanowire MOSFET, a dummy gate is generally formedas a sacrificial structure to facilitate patterning to achieve a desiredalignment and/or the implantation of ions for doping purposes. Areplacement gate flow is then used to remove the dummy gate and installa permanent replacement gate. In the replacement gate flow, the dummygate is patterned. The nanowire is then released from the dummy gate bythe removal of the dummy gate. In removing the dummy gate, the materialunderneath the nanowire is undercut to cause the complete release of thewire from the material of the dummy gate and to form an opening that canbe filled with a GAA structure. However, the undercutting also extendsthe opening in the directions toward the source and drain, therebyadding to the parasitic capacitance of the MOSFET.

BRIEF SUMMARY

In one exemplary aspect, a method for fabricating a semiconductor devicecomprises forming a nanowire on an insulator layer at a surface of asubstrate; forming a dummy gate over a portion of the nanowire and aportion of the insulator layer; forming recesses in the insulator layeron opposing sides of the dummy gate; forming spacers on opposing sidesof the dummy gate; forming source regions and drain regions in therecesses in the insulator layer on opposing sides of the dummy gate;depositing an interlayer dielectric on the source regions and the drainregions; removing the dummy gate to form a trench; removing theinsulator layer under the nanowire such that a width of the trenchunderneath the nanowire is equal to or less than a distance between thespacers; and forming a replacement gate in the trench.

In another exemplary aspect, a method for fabricating a semiconductordevice comprises forming a dummy gate stack over a portion of andsubstantially transverse to a nanowire on a silicon-on-insulator wafer;forming recesses in the silicon-on-insulator wafer on opposing sides ofthe dummy gate stack; forming spacers in the recesses on opposing sidesof the dummy gate stack; forming source regions and drain regions onopposing sides of the spacers; depositing an interlayer dielectric onthe source regions and the drain regions; planarizing an upper surfacedefined by the interlayer dielectric, upper edges of the spacers, and anupper surface of the dummy gate stack; removing the dummy gate stack;removing a portion of the silicon-on-insulator wafer underneath thedummy gate stack, underneath the nanowire, and between inner facingsurfaces of the spacers; and forming a replacement gate in place of thedummy gate stack, the replacement gate extending at least under thenanowire to form a gate-all-around structure and extending between theinner facing surfaces of the spacers.

In another exemplary aspect, an apparatus for a semiconductor devicecomprises a nanowire formed on an insulator layer disposed on asubstrate of semiconductor material, a first end of the nanowire beingin communication with a source region and a second end of the nanowirebeing in communication with a drain region; and a gate positioned acrossand extending substantially transverse to the nanowire between thesource region and the drain region, the gate comprising an electrode andspacers positioned on opposing sides of the electrode and extendingsubstantially transverse to the nanowire. A bottom portion of the gatesurrounds a portion of the nanowire extending from the source region tothe drain region, and outer surfaces of the gate extend verticallydownward into the insulator layer but do not extend beyond the innerfaces of the spacers toward the source region and the drain region.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The foregoing and other aspects of the exemplary embodiments are mademore evident in the following Detailed Description, when read inconjunction with the attached Drawing Figures, wherein:

FIG. 1 is a top sectional view of one exemplary embodiment of anapparatus incorporating GAA nanowire architecture in which a gate ispositioned transverse to a nanowire;

FIG. 2A is a side view along cross section A-A′ of FIG. 1 during a stepin the fabrication of the apparatus;

FIG. 2B is a side view along cross section B-B′ of FIG. 1 during a stepin the fabrication of the apparatus;

FIG. 3A is a side view along cross section A-A′ during a step of forminga dummy gate stack over a nanowire in the fabrication of the apparatus;

FIG. 3B is a side view along cross section B-B′ during the step offorming the dummy gate stack in the fabrication of the apparatus;

FIG. 4A is a side view along cross section A-A′ during a step of etchingan insulator layer in the fabrication of the apparatus;

FIG. 4B is a side view along cross section B-B′ during the step ofetching the insulator layer in the fabrication of the apparatus;

FIG. 5A is a side view along cross section A-A′ during a step of forminga spacer on the dummy gate stack in the fabrication of the apparatus;

FIG. 5B is a side view along cross section B-B′ during the step offorming the spacer on the dummy gate stack in the fabrication of theapparatus;

FIG. 6A is a side view along cross section A-A′ during a step of formingsource/drain regions in the fabrication of the apparatus;

FIG. 6B is a side view along cross section B-B′ during the step offorming source/drain regions in the fabrication of the apparatus;

FIG. 7A is a side view along cross section A-A′ during a step ofdepositing an interlayer dielectric and applying a chemical mechanicalpolish in the fabrication of the apparatus;

FIG. 7B is a side view along cross section B-B′ during the step ofdepositing the interlayer dielectric and applying the chemicalmechanical polish in the fabrication of the apparatus;

FIG. 8A is a side view along cross section A-A′ during a step ofremoving the dummy gate in the fabrication of the apparatus;

FIG. 8B is a side view along cross section B-B′ during the step ofremoving the dummy gate in the fabrication of the apparatus;

FIG. 9A is a side view along cross section A-A′ during a step ofundercutting the nanowire in the fabrication of the apparatus;

FIG. 9B is a side view along cross section B-B′ during the step ofremoving portions of the insulator layer in the fabrication of theapparatus;

FIG. 10A is a side view along cross section A-A′ during a step offorming a replacement gate in the fabrication of the apparatus;

FIG. 10B is a side view along cross section B-B′ during the step offorming the replacement gate in the fabrication of the apparatus;

FIG. 11A is a cross sectional side view of a prior art apparatus takenalong a nanowire in which during undercutting of an insulating layer,the insulating layer was laterally etched underneath spacers of theapparatus;

FIG. 11B is a cross sectional side view of the prior art apparatus ofFIG. 11A in which during undercutting of the insulating layer, theinsulating layer was laterally etched underneath spacers of theapparatus;

FIG. 12A is a side view along cross section A-A′ during a step offorming a second exemplary embodiment of an apparatus in which thesource/drain regions undercut the gate stack and the nanowire;

FIG. 12B is a side view along cross section B-B′ during the step offorming the second exemplary embodiment of the apparatus in which thesource/drain regions undercut the gate stack;

FIG. 13A is a side view along cross section A-A′ during a step ofextending the spacers under the nanowire;

FIG. 13B is a side view along cross section B-B′ during the step ofextending the spacers under the gate stack; and

FIG. 14 is a flow of a method of forming a GAA nanowire structure.

DETAILED DESCRIPTION

In the exemplary embodiments disclosed herein, GAA nanowire architectureis used in the fabrication of a CMOS such as a MOSFET to allow forimproved electrostatic gate control in a conducting channel and to offerthe potential to drive more current per device area than is possible inconventional planar CMOS architectures. In implementing the GAA nanowirearchitecture, a dummy gate comprising an oxide material is formed over ananowire so as to extend substantially transverse to the nanowire. Lowerportions of the dummy gate extend below and underneath the nanowire andinto a surface on which the nanowire is disposed. Upper portions of thematerial of the dummy gate are removed, and lower portions of thematerial of the dummy gate (the oxide under the nanowire extendingbetween source and drain regions) are recessed and undercut. Spacers arethen formed on opposing sides of the dummy gate. When the oxide isundercut after removal of the upper portions of the dummy gate, thelateral extent of the opening is limited by the presence of the spacers.

Referring to FIG. 1, a GAA nanowire structure for a MOSFET is designatedgenerally by the reference number 100 and is hereinafter referred to as“apparatus 100.” Apparatus 100 comprises a nanowire 110 generallylaterally disposed on an insulator layer 120, the insulator layer 120being disposed on a substrate (shown at 130 in FIGS. 2A-10B). Crosssection A-A′ is parallel to and coincident with the nanowire 110, andcross section B-B′ is parallel to the nanowire 110 and between thenanowire 110 and a second nanowire (not shown) positioned parallel toand distal from the nanowire 110. In the finished apparatus 100, theinsulator layer 120 may be a buried oxide (BOX) layer. In an exemplaryembodiment, the nanowire 110 may be epitaxially grown on the insulatorlayer 120 using chemical vapor deposition (CVD) or any other suitablemethod. Source regions 220 and drain regions 230 are deposited on andadjacent to the nanowire 110 by epitaxy.

Referring now to FIG. 2A, one or more patterns are laid out for theepitaxial growth (or other formation) of nanowires 110 as channels onthe insulator layer 120 of a silicon-on-insulator (SOI) wafer. Thenanowire 110 is patterned onto the insulator layer 120 by any suitablemethod. Methods by which the nanowire may be patterned onto theinsulator layer 120 include, but are not limited to, lithographictechniques such as atomic force microscope (AFM) nano-oxidation andselective wet etching, E-beam lithography, and X-ray lithography.

The SOI wafer is defined by the substrate 130 and the insulator layer120. The substrate 130 may comprise any semiconducting material such as,for example, silicon carbide (SiC), silicon alloys, germanium, germaniumalloys, alloys of silicon and germanium (Si_(x)Ge_(y)), gallium arsenide(GaAs), indium arsenide (InAs), indium phosphide (InP), and the like.The insulator layer 120 may comprise, for example, silicon dioxide(SiO₂) or the like.

The one or more nanowires 110 are then epitaxially grown to form thechannels. The nanowire 110 may comprise silicon, germanium, boron-dopedgermanium, or any other suitable material.

As shown in FIG. 2B, a nanowire is not patterned onto the insulatorlayer 120 on a portion of the insulator layer 120 along the crosssection B-B′ parallel, to the nanowire 110 and between the nanowire 110and a second nanowire.

Referring now to FIGS. 3A and 3B, a dummy gate stack 140 is formed bothon the nanowire 110 (FIG. 3A) and on the insulator layer 120 (FIG. 3B).The dummy gate stack 140 may be substantially transverse to the nanowire110. In forming the dummy gate stack 140 across the nanowire 110, adielectric layer 150 is deposited or grown, on a surface of the nanowire110. This dielectric layer 150 comprises, for example, SiO₂, alumina(Al₂O₃), tantalum pentoxide (Ta₂O₅), hafnium dioxide (BfO₂), or thelike, deposited onto the nanowire 110 by, for example, thermaloxidation, atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD),sputtering techniques, or electron-beam deposition.

Subsequent to the deposition or growth of the dielectric layer 150, adummy gate electrode 160 is deposited onto the dielectric layer 150 andonto the insulator layer 120. The dummy gate electrode 160 may comprise,for example, polysilicon (or metal or a combination of polysilicon andmetal) and may be deposited via LPCVD or any other suitable method.

Subsequent to the deposition of the dummy gate electrode 160, a dummygate cap 170 is deposited onto the dummy gate electrode 160. The dummygate cap 170 may comprise, for example, a nitride (silicon nitride,carbon nitride, or the like) or a silicide and may be deposited viaLPCVD.

Referring now to FIGS. 4A and 4B, the material of the insulator layer120 (e.g., the SiO₂) is etched vertically. As shown in FIG. 4A, thedummy gate stack 140 and the nanowire 110 are used as a mask to preventthe etching of the insulator layer 120 underneath the dummy gate stack140 and the nanowire 110. As shown in FIG. 4B, the material of theinsulator layer 120 is removed by the etch to form recesses 180. Etchingof the insulator layer 120 to form the recesses 180 may be effected bywet etch techniques (e.g., polyphosphoric acids, hydrofluoric acid (HF),potassium hydroxide, or the like) or plasma or gas etch techniques.Irrespective of the technique used to etch, the material of theinsulator layer 120 is recessed vertically from the outer surfaces ofthe dummy gate stack 140 such that the insulator layer 120 directlyunder the dummy gate 120 is not removed.

Once the recesses 180 are formed in the insulator layer 120, spacers 200are formed on opposing sides of the dummy gate stack 140, as shown inFIGS. 5A and 5B. Along the cross section A-A′, the spacers 200 extendfrom the top of the dummy gate cap 170 to the nanowire 110. Between thenanowire 110 and adjacent nanowires, however, the spacers 200 extend toa depth below the bottom of the nanowire 110 and the dummy gate stack140 (to the bottom surfaces defined by the recesses 180). The spacers200 are formed by masking off the nanowire 110 and the insulator layer120 and depositing dielectric material, such as SiO₂, silicon nitride,low-permittivity (low-K) dielectrics such as SiO_(x)N_(y) or boronnitride, via CVD or LPCVD.

Referring now to FIGS. 6A and 6B, a source and a drain may be formed bya selective epitaxial growth process such as vapor phase epitaxy, whichis a form of CVD. Epitaxial growth refers to the deposition of acrystalline overlayer on a crystalline sub-layer where the structure ofthe overlayer registers with the structure of the crystalline sub-layer.As can be seen in FIG. 6A, a layer of source material and drain materialis epitaxially grown on opposing sides of the dummy gate stack 140 andover the nanowire 110 to form the source regions 220 and drain regions230. As can be seen in FIG. 6B, a layer of the material used to grow thesource regions 220 and the drain regions 230 is also epitaxially grownon the insulator layer 120 at opposing sides of the dummy gate stack 140and over the cross section B-B′ parallel to the nanowire 110 and betweenthe nanowire 110 and a second nanowire.

As shown in FIGS. 7A and 7B, an interlayer dielectric (ILD) 250 isdeposited onto the epitaxially grown source regions 220 and drainregions 230 by CVD. The resulting structure is then planarized using achemical mechanical polish (CMP). Materials from which the ILD 250 maybe formed include oxides such as SiO₂.

Referring now to FIGS. 8A and 8B, the dummy gate cap 170 and the dummygate electrode 160 are removed in an etching process that comprises oneor more of a dry etch process (e.g., a reactive ion etch (RIE), plasmaetching, or the like) and a wet etch process using phosphoric acid(H₃PO₄). Preferably, the dummy gate cap 170 is removed using the dryetch process. Once the dummy gate cap 170 is removed, the dummy gateelectrode 160 is removed using the RIE. Top portions of the spacers 200are removed down to the bottom surface of the dummy gate cap 170, butbottom portions of the spacers 200 remain in the ILD 250. Etching in thevertical direction is then carried out along with an isotropic etch(such as aqueous HF, dilute aqueous HF, HF vapor, buffered oxide etch(e.g., NH₄F:HF), or the like) to remove the dielectric layer 150 (theoxide) and portions of the insulator layer 120 down to a depth at orbelow a bottom surface of the nanowire 110 and between the spacers. Thedry etch process and/or the wet etch process along with the isotropicetch forms a trench 260 that extends in the insulator layer 120transverse to cross section A-A′ and cross section B-B′ and leavesportions of the nanowire(s) 110 extending between the spacer 200exposed.

As shown in FIG. 9A, further isotropic etching results in additionalinsulator layer 120 being removed and the nanowire 110 being undercutsuch that the bottom portion of the nanowire 110 is released from thesource regions 220 and the drain regions 230. Thus, the nanowire 110 isessentially suspended between the source region 220 and the drain region230. The sides of the opening formed under the nanowire 110 by theisotropic etch are substantially coplanar with the inner facing surfacesof the spacers 200, which are formed before the nanowire is suspendedbetween the source regions 220 and the drain regions 230. As shown inFIG. 9B, lateral undercutting of the insulator layer 120 is prevented orat least limited by the spacers 200.

Referring now to FIGS. 10A and 10B, a replacement gate 300 is formed inthe space between the spacers 200. In a first step in forming thereplacement gate 300 (which may be a replacement metal gate (RMG)), alayer 310 of high-k dielectric material is deposited on the surfacesdefining the opening between the spacers 200 as well as on nanowire 110(FIG. 10A). This layer 310 of high-k dielectric material may bedeposited by, for example, CVD. The high-k dielectric material may be,for example, oxide(s) of tantalum, zirconium, or aluminum, as well asSiO₂ or Al₃N₄.

In a second step of forming the replacement gate 300, a gate metal isdeposited on the layer 310 of high-k dielectric material. The gate metalis preferably aluminum, nickel, tantalum, tantalum nitride, titanium,titanium nitride, TiAl alloy, ruthenium, tungsten, or the like and isdeposited on the high-k dielectric material via vapor deposition.Formation of the replacement gate 300 as indicated is different than theformation of similar gates in the known art, as shown in FIGS. 11A and11B, in which portions of a gate 330 and/or a layer 340 of dielectricmaterial extend laterally underneath spacers 200 by a distance d. In theformation of the replacement gate 300 as in FIGS. 10A and 10B (andsimilar gates) to form the GAA structure, the oxide layer is laterallyetched during the undercutting process. Compared to the gate 330 asshown in FIGS. 11A and 11B, the portion of the replacement gate 300under the nanowire 110 and between the nanowires is narrower.

In another exemplary embodiment, as shown in FIGS. 12A and 12B, thematerial of the source regions 220 and the drain regions 230 issubjected to a vertical etching process along with an isotropic etchingprocess (oxide undercutting) carried out in lateral directions to formrecesses (shown at 400) underneath the dummy gate stack 140 as well asrecesses (shown at 410) underneath the nanowire 110 in outwarddirections toward the source region 220 and the drain regions 230.Depending upon the extent of the undercutting of dummy gate stack 140,portions of the nanowire 110 (FIG. 12A) may or may not be completelyreleased.

Referring now to FIGS. 13A and 13B, lower portions of the spacers 200along cross section A-A′ are extended under the nanowire 110 (FIG. 13A)both inwardly of the gate region to fill the recesses 400 as well as inthe outward directions underneath the nanowire 110 toward the sourceregion 220 and the drain region 230. Lower portions of the spacers 200between nanowires (e.g., along cross section B-B′) are laterallyextended into the recesses 400. The layer 310 of high-k dielectricmaterial is deposited, and the replacement gate 300 is formed on thelayer 310 of high-k dielectric material. In such an embodiment, portionsof the replacement gate 300 under the nanowire 110 both directly underthe nanowire 110 and in the spaces between nanowires are narrower ascompared to gates in the known art.

Referring now to FIG. 14, one exemplary a method of forming a GAAnanowire structure is designated generally by the reference number 1400and is hereinafter referred to as “method 1400.” Method 1400 comprises aproviding step 1405 in which an insulator layer is generally provided ona substrate. After the providing step 1405, a nanowire is patterned ontothe insulator layer in a patterning step 1410. A dielectric layer isthen deposited onto the nanowire in a dielectric layer deposit step1415. A dummy gate electrode is then deposited onto the dielectric layerin a dummy gate electrode deposit step 1420. A dummy gate cap is thendeposited onto the dummy gate electrode in a dummy gate cap deposit step1425. The insulator layer is then etched in an insulator layer etch step1430, and spacers are formed on opposing sides of the dummy stack in aspacer formation step 1435. Source and drain regions are then formed onopposing sides of the dummy stack in a source/drain region formationstep 1440. An ILD layer is deposited onto the source/drain regions in anILD deposit step 1445, and the ILD is planarized in a planarize step1450. The dummy gate cap and dummy gate electrode are then removed in aremoval step 1455, and the nanowire is undercut in an undercutting step1460 to cause the nanowire to be suspended between the source/drainregions. Following the undercutting step 1460, a replacement gate isformed in a replacement gate formation step 1465.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The description of the present invention has been presented for purposesof illustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiments were chosen and described in order to best explain theprinciples of the invention and the practical applications, and toenable others of ordinary skill in the art to understand the inventionfor various embodiments with various modifications as are suited to theparticular uses contemplated.

1. A gate-all-around structure for a semiconductor device, comprising: asubstrate; an insulator layer comprising silicon dioxide disposed on thesubstrate; a nanowire substantially laterally disposed on the insulatorlayer; a source region disposed in communication with a first end of thenanowire; a drain region disposed in communication with a second end ofthe nanowire; a gate positioned substantially transverse to thenanowire, a bottom portion of the gate surrounding a portion of thenanowire between the source region and the drain region, and wherein thewidth of the bottom portion of the gate is less than the width of a topportion of the gate; and a layer of high-k dielectric material disposedon the nanowire between the gate and the nanowire.
 2. The structure ofclaim 1, further comprising spacers positioned on opposing sides of thegate.
 3. The structure of claim 2, wherein inner facing surfaces onlower portions of the spacers extend laterally inward.
 4. The structureof claim 3, wherein outward facing surfaces on lower portions of thespacers extend laterally outward underneath the nanowires.
 5. Thestructure of claim 2, wherein the spacers comprise SiO₂, siliconnitride, SiO_(x)N_(y), or boron nitride.
 6. The structure of claim 1,wherein the gate is a replacement metal gate.
 7. The structure of claim1, wherein the gate comprises a gate metal disposed on the layer ofhigh-k dielectric material.
 8. The structure of claim 7, wherein thegate metal comprises aluminum, nickel, tantalum, tantalum nitride,titanium, titanium nitride, TiAl alloy, ruthenium, or tungsten.
 9. Thestructure of claim 1, wherein the high-k dielectric material is tantalumoxide, zirconium oxide, aluminum oxide, SiO₂, or Al₃N₄.
 10. Thestructure of claim 1, wherein the substrate and the insulator layerdefine a silicon-on-insulator wafer.
 11. The structure of claim 1,wherein the substrate comprises silicon carbide, a silicon alloy,germanium, germanium alloy, alloy of silicon and germanium, galliumarsenide, indium arsenide, or indium phosphide.
 12. The structure ofclaim 1, further comprising an interlayer dielectric material disposedon the source region and the drain region.
 13. A gate-all-aroundstructure for a semiconductor device, comprising: a substrate having aninsulator layer comprising silicon dioxide disposed thereon; a nanowiresubstantially laterally disposed on the insulator layer; a source regiondisposed in communication with a first end of the nanowire; a drainregion disposed in communication with a second end of the nanowire; alayer of high-k dielectric material disposed on the nanowire; and a gatepositioned substantially transverse to the nanowire and on the layer ofhigh-k dielectric material and spacers positioned on opposing sides ofthe gate, and a bottom portion of the gate surrounding a portion of thenanowire between the source region and the drain region, wherein thebottom portion of the gate under the nanowire is narrower than a topportion of the gate above the nanowire.
 14. The structure of claim 13,wherein inner facing surfaces at bottom portions of the spacers extendinward into the gate.
 15. The structure of claim 14, wherein outwardfacing surfaces at bottom portions of the spacers extend outwardunderneath the nanowires.
 16. The structure of claim 13, wherein thehigh-k dielectric material disposed on the nanowire is tantalum oxide,zirconium oxide, aluminum oxide, SiO₂, or Al₃N₄.
 17. The structure ofclaim 13, wherein the gate comprises aluminum, nickel, tantalum,tantalum nitride, titanium, titanium nitride, TiAl alloy, ruthenium, ortungsten.
 18. The structure of claim 13, wherein the insulator layer isa buried oxide layer.
 19. The structure of claim 13, further comprisingan interlayer dielectric material disposed on the source region and thedrain region.